Angle-resolved Photoemission Spectroscopy Research Articles

Non-Hermitian Boundary in a Surface Selective Reconstructed Magnetic Weyl Semimetal.

Non-Hermitian physics, studying systems described by non-Hermitian Hamiltonians, reveals unique phenomena not present in Hermitian systems. Unlike Hermitian systems, non-Hermitian systems have complex eigenvalues, making their effects less directly observable. Recently, significant efforts have been devoted to incorporating the non-Hermitian effects into condensed matter physics. However, progress is hindered by the absence of a viable experimental approach. Here, the discovery of the surface-selectively spontaneous reconstructed Weyl semimetal NdAlSi provides a feasible experimental platform for studying non-Hermitian physics. Utilizing angle-resolved photoemission spectroscopy (ARPES) measurements, surface-projected density functional theory (DFT) calculations, and scanning tunneling microscopy (STM) measurements, it is demonstrated that surface reconstruction in NdAlSi alters surface Fermi arc (SFA) connectivity and generates new isolated non-topological SFAs (NTSFAs) by introducing non-Hermitian terms. The surface-selective spontaneous reconstructed Weyl semimetal NdAlSi can be viewed as a Hermitian bulk - non-Hermitian boundary system. The isolated non-topological SFAs on the reconstructed surface act as a loss mechanism and open boundary condition (OBC) for the topological electrons and bulk states, serving as non-Hermitian boundary states. This discovery provides a good experimental platform for exploring new physical phenomena and potential applications based on boundary non-Hermitian effects, extending beyond purely mathematical concepts. Furthermore, it provides important enlightenment for constructing topological photonic crystals with surface reconstruction and studying their topologicalproperties.

Anisotropic Valence Band Structure of Anatase TiO2(101) Studied by Polarization-Dependent Angle-Resolved Photoelectron Spectroscopy

Anisotropic Valence Band Structure of Anatase TiO₂(101) Studied by Polarization-Dependent Angle-Resolved Photoelectron Spectroscopy

Folded pseudochiral Fermi surface in 4Hb-TaSe2 from band hybridization with a charge density wave

Abstract Stacking of strongly-correlated 2D materials is opening the possibility to demonstrate novel electronic or magnetic ordering phenomena. In this regard the intrinsic polytypism of tantalum dichalcogenides has emerged as a platform to generate clean and controllable material interfaces. Here, we report on the Fermi surface of 4Hb-TaSe2, a polytype which consists of alternately stacked layers with octahedral (T) and trigonal prismatic (H) coordination of tantalum in the Se-Ta-Se layers. The material is known to host a charge density wave (CDW) phase with star clusters in the T-layers, intercalated by metallic H-layers, but its momentum resolved electronic structure remains undetermined. Using selective area angle resolved photoemission spectroscopy on the T termination combined with ab initio calculations, we unveil a finely structured Fermi surface arising from band folding in the reconstructed Brillouin zone caused by the CDW star clusters. The star-shaped Fermi surface is rotated away from the high-symmetry directions of the normal phase, and exhibits pseudochirality. Theoretical analysis supports the metallic nature of the system and interlayer interactions leading to hybridization. The work provides a detailed overview on the impact of band hybridization with the CDW on the Fermi surface of a material for new phases of quantum matter.

Coalescence of multiple topological orders in quasi-one-dimensional bismuth halide chains

Topology is being widely adopted to understand and to categorize quantum matter in modern physics. The nexus of topology orders, which engenders distinct quantum phases with benefits to both fundamental research and practical applications for future quantum devices, can be driven by topological phase transition through modulating intrinsic or extrinsic ordering parameters. The conjoined topology, however, is still elusive in experiments due to the lack of suitable material platforms. Here we use scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and theoretical calculations to investigate the doping-driven band structure evolution of a quasi-one-dimensional material system, bismuth halide, which contains rare multiple band inversions in two time-reversal-invariant momenta. According to the unique bulk-boundary correspondence in topological matter, we unveil a composite topological phase, the coexistence of a strong topological phase and a high-order topological phase, evoked by the band inversion associated with topological phase transition in this system. Moreover, we reveal multiple-stage topological phase transitions by varying the halide element ratio: from high-order topology to weak topology, the unusual dual topology, and trivial/weak topology subsequently. Our results not only realize an ideal material platform with composite topology, but also provide an insightful pathway to establish abundant topological phases in the framework of band inversion theory.

An autoencoder for compressing angle-resolved photoemission spectroscopy data

Abstract Angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental technique to determine the electronic structure of solids. Advances in light sources for ARPES experiments are currently leading to a vast increase of data acquisition rates and data quantity. On the other hand, access time to the most advanced ARPES instruments remains strictly limited, calling for fast, effective, and on-the-fly data analysis tools to exploit this time. In response to this need, we introduce ARPESNet, a versatile autoencoder network that efficiently summmarises and compresses ARPES datasets. We train ARPESNet on a large and varied dataset of 2-dimensional ARPES data extracted by cutting standard 3-dimensional ARPES datasets along random directions in k. To test the data representation capacity of ARPESNet, we compare k-means clustering quality between data compressed by ARPESNet, data compressed by discrete cosine transform, and raw data, at different noise levels. ARPESNet data excels in clustering quality despite its high compression ratio.

Large exciton binding energy in a bulk van der Waals magnet from quasi-1D electronic localization

Excitons, bound electron-hole pairs, influence the optical properties in strongly interacting solid-state systems and are typically most stable and pronounced in monolayer materials. Bulk systems with large exciton binding energies, on the other hand, are rare and the mechanisms driving their stability are still relatively unexplored. Here, we report an exceptionally large exciton binding energy in single crystals of the bulk van der Waals antiferromagnet CrSBr. Utilizing state-of-the-art angle-resolved photoemission spectroscopy and self-consistent ab-initio GW calculations, we present direct spectroscopic evidence supporting electronic localization and weak dielectric screening as mechanisms contributing to the amplified exciton binding energy. Furthermore, we report that surface doping enables broad tunability of the band gap offering promise for engineering of the optical and electronic properties. Our results indicate that CrSBr is a promising material for the study of the role of anisotropy in strongly interacting bulk systems and for the development of exciton-based optoelectronics.

Anisotropic temperature dependence of the electronic band structure in black phosphorus

Abstract Black phosphorus (BP) has attracted much attention because of its anisotropic layer-dependent optical and electronic structure and ultrahigh carrier mobility. Despite the importance of manipulating the optical and electronic properties of BP by temperature variation, the response of the electronic band structure to temperature and the underlying mechanism which are crucial to understand the related applications haven’t been reported yet. Herein, temperature dependence of the electronic band structure of BP has been measured by angle-resolved photoelectron spectroscopy (ARPES) and the band dispersions are quantitatively characterized. While the hole effective mass along k x (m x ) decreases with temperature, that along k y (m y ) increases with temperature. We argue that the electronic band dispersion along k x is decreased because it is dictated by the weakened interlayer and intralayer couplings, and the increase of m y with temperature is ascribed to the enhanced electron–phonon scattering. This work reports the anisotropic temperature dependence of the electronic band dispersion of BP and reveals the underlying mechanisms, which will expand the knowledge of electronic structure and benefit the understanding of temperature dependent phenomena in this material.

Flat Band Generation Through Interlayer Geometric Frustration in Intercalated Transition Metal Dichalcogenides.

Electronic flat bands can lead to rich many-body quantum phases by quenching the electron's kinetic energy and enhancing many-body correlation. The reduced bandwidth can be realized by either destructive quantum interference in frustrated lattices, or by generating heavy band folding with avoided band crossing in Moiré superlattices. Here a general approach is proposed to introduce flat bands into widely studied transition metal dichalcogenide (TMD) materials by dilute intercalation. A flat band with vanishing dispersion is observed by angle-resolved photoemission spectroscopy (ARPES) over the entire momentum space in intercalated Mn1/4TaS2. Polarization-dependent ARPES measurements combined with symmetry analysis reveal the orbital characters of the flat band. Supercell tight-binding simulations suggest that such flat bands arising from destructive interference between Mn and Ta on S through hopping pathways, are ubiquitous in a range of TMD families as well as for different intercalation configurations. The findings establish a new material platform to manipulate flat band structures and explore their corresponding emergent correlated properties.

Reversal of charge transfer doping on the negative electronic compressibility surface of MoS2

Abstract The strong electron-electron interaction in transition metal dichalcogenides (TMDs) gives rise to phenomena such as strong exciton and trion binding and excitonic condensation, as well as large negative exchange and correlation contributions to the electron energies, resulting in negative electronic compressibility. Here we use angle-resolved photoemission spectroscopy to demonstrate a striking effect of negative electronic compressibility in semiconducting TMD MoS2 on the charge transfer to and from a partial overlayer of monolayer semimetallic WTe2. By systematically monitoring the binding energy shifts in the valence bands of both WTe2 and MoS2 during surface transfer doping with donor (K) and acceptor (F4-TCNQ) species, we observe distinct behaviors: (1) for donor doping, increased MoS2 valence band binding energy is accompanied by a counterintuitive reduction in the binding energy of WTe2 valence bands and core levels; (2) for acceptor doping, the expected decrease in MoS2 binding energies contrasts with an unexpected increase in those of WTe2. The observations imply a reversal of the expected charge transfer; donor (acceptor) deposition decreases (increases) the carrier density in the WTe2 adlayer. The charge transfer reversal is a direct consequence of the negative electronic compressibility of the MoS2 surface layer, for which addition (subtraction) of charge leads to attraction (repulsion) of further charge from neighbouring layers. These findings highlight the importance of many-body interactions for the electrons in transition metal dichalcogenides and underscore the potential for exploring strongly correlated quantum states in two-dimensional semiconductors.

Electronic structure of superconducting infinite-layer lanthanum nickelates.

Revealing the momentum-resolved electronic structure of infinite-layer nickelates is essential for understanding this class of unconventional superconductors but has been hindered by the formidable challenges in improving the sample quality. In this work, we report the angle-resolved photoemission spectroscopy of superconducting La0.8Sr0.2NiO2 films prepared by molecular beam epitaxy and in situ atomic-hydrogen reduction. The measured Fermi topology closely matches theoretical calculations, showing a large Ni [Formula: see text]-derived Fermi sheet that evolves from hole-like to electron-like along kz and a three-dimensional (3D) electron pocket centered at the Brillouin zone corner. The Ni [Formula: see text]-derived bands show a mass enhancement (m*/mDFT) of 2 to 3, while the 3D electron band shows negligible band renormalization. Moreover, the Ni [Formula: see text]-derived states also display a band dispersion anomaly at higher binding energy, reminiscent of the waterfall feature and kinks observed in cuprates.

Unveiling a Tunable Moiré Bandgap in Bilayer Graphene/hBN Device by Angle-Resolved Photoemission Spectroscopy.

Over the years, great efforts have been devoted in introducing a sizable and tunable band gap in graphene for its potential application in next-generation electronic devices. The primary challenge in modulating this gap has been the absence of a direct method for observing changes of the band gap in momentum space. In this study, advanced spatial- and angle-resolved photoemission spectroscopy technique is employed to directly visualize the gap formation in bilayer graphene, modulated by both displacement fields and moiré potentials. The application of displacement field via in situ electrostatic gating introduces a sizable and tunable electronic bandgap, proportional to the field strength up to 100 meV. Meanwhile, the moiré potential, induced by aligning the underlying hexagonal boron nitride substrate, extends the bandgap by ≈20 meV. Theoretical calculations effectively capture the experimental observations. This investigation provides a quantitative understanding of how these two mechanisms collaboratively modulate the band gap in bilayer graphene, offering valuable guidance for the design of graphene-based electronic devices.

Evaluation of the effect of hydrogen plasma treatment on the chemical bonding state and distribution of hydrogen in silicon nitride films by angle-resolved hard X-ray photoelectron spectroscopy

Abstract The presence of hydrogen-incorporated defects in silicon nitride (SiN) films has been reported to degrade device properties, such as NAND flash memory. Therefore, controlling the amount of hydrogen defects in SiN films to achieve high device reliability is important. This study investigated hydrogen desorption using hydrogen plasma treatment to reduce hydrogen-incorporated defects in SiN films. Secondary ion mass spectrometry measurements showed that the hydrogen concentration in SiN films does not change significantly pre- and post-treatment. However, the trap levels in SiN films determined using the Poole–Frenkel analysis of hole currents showed a clear deepening after the plasma treatment. Angle-resolved hard X-ray photoelectron spectroscopy measurements suggested that hydrogen plasma treatment decreases the ratio of silicon to hydrogen or nitrogen to hydrogen bonds as a function of treatment time.

Role of spin-orbit coupling for the band splitting in ⍺-Sb and ⍺-Bi on SiC(0001).

Monolayer atomic thin films of group-V elements have a high potential for application in spintronics and valleytronics because of their unique crystal structure and strong spin-orbit coupling. We fabricated Sb and Bi monolayers on a SiC(0001) substrate by the molecular-beam-epitaxy method and studied the electronic structure by angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. The fabricated Sb film shows the (√3×√3)R30º superstructure associated with the formation of ⍺-Sb, and exhibits a semiconducting nature with a band gap of more than 1.8 eV. Spin-resolved ARPES measurements of isostructural ⍺-Bi revealed the in-plane spin polarization for the topmost valence band, demonstrating its Rashbasplittingof nature due to the space-inversion-symmetry breaking. We discuss the originobserved characteristic band structure and its similarity and difference between Sband Bi.&#xD.

Molecular Order Induced Charge Transfer in a C60-Topological Insulator Moiré Heterostructure.

We synthesized and spectroscopically investigated monolayer (ML) C60 on the topological insulator (TI) Bi4Te3. This C60/Bi4Te3 heterostructure is characterized by an excellent translational order in a novel (4 × 4) C60 superstructure on a (9 × 9) cell of Bi4Te3. Angle-resolved photoemission spectroscopy (ARPES) of C60/Bi4Te3 reveals that ML C60 accepts electrons from the TI at room temperature, but no charge transfer occurs at low temperatures. This temperature-dependent doping is further investigated by Raman spectroscopy, photoluminescence (PL), and calculations of C60/Bi4Te3. At low temperatures, Raman spectroscopy and PL show a dramatic intensity increase of the C60-related signal, suggesting a transition to a rotationally ordered state. Calculations explain the charge transfer by C60 adsorption to Bi4Te3 surface defects. The temperature dependence of the charge transfer is attributed to the orientational order of C60. The electron affinity of C60 increases at low temperatures due to the freezing of the rotational motion.

In-Plane Anisotropy in van der Waals NiTeSe Ternary Alloy.

The anisotropic properties of materials profoundly influence their electronic, magnetic, optical, and mechanical behaviors and are critical for a wide range of applications. In this study, the anisotropic characteristics of Ni-based van der Waals materials, specifically NiTe2 and its alloy NiTeSe, utilizing a combination of comprehensive scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations, are explored. Unlike 1T-NiTe2, which exhibits trigonal in-plane symmetry, the substitution of Te with Se in NiTe2 (resulting in the NiTeSe alloy) induces a pronounced in-plane anisotropy. This anisotropy is clear in the STM topographs, which reveal a distinct linear order of charge distribution. Corroborating these observations, ARPES measurements and DFT calculations reveal an anisotropic Fermi surface centered at the point, which is notably elongated along the ky direction, leading to directional variations in in-plane carrier velocities. Consequently, the Fermi velocity is highest along the kx direction where the linear charge distribution aligns in real space and is lowest along the ky direction. These findings offer valuable insights into the tunability of anisotropic properties in ternary transition metal dichalcogenide systems, highlighting their potential applications in the development of anisotropic electronic and optoelectronic devices.

Role of spin-orbit coupling for the band splitting in ⍺-Sb and ⍺-Bi on SiC(0001).

Monolayer atomic thin films of group-V elements have a high potential for application in spintronics and valleytronics because of their unique crystal structure and strong spin-orbit coupling. We fabricated Sb and Bi monolayers on a SiC(0001) substrate by the molecular-beam-epitaxy method and studied the electronic structure by angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. The fabricated Sb film shows the (√3×√3)R30º superstructure associated with the formation of ⍺-Sb, and exhibits a semiconducting nature with a band gap of more than 1.8 eV. Spin-resolved ARPES measurements of isostructural ⍺-Bi revealed the in-plane spin polarization for the topmost valence band, demonstrating its Rashbasplittingof nature due to the space-inversion-symmetry breaking. We discuss the originobserved characteristic band structure and its similarity and difference between Sband Bi.&#xD.

First-Principles Assessment of ZnTe and CdSe as Prospective Tunnel Barriers at the InAs/Al Interface.

Majorana zero modes are predicted to emerge in semiconductor/superconductor interfaces, such as InAs/Al. Majorana modes could be utilized for fault tolerant topological qubits. However, their realization is hindered by materials challenges. The coupling between the superconductor and the semiconductor may be too strong for Majorana modes to emerge, due to effective doping of the semiconductor by the metallic contact. This could be mediated by adding a tunnel barrier of controlled thickness. We use density functional theory (DFT) with Hubbard U corrections, whose values are machine-learned via Bayesian optimization (BO), to assess ZnTe and CdSe as prospective tunnel barriers for the InAs/Al interface. The results of DFT+U(BO) for ZnTe are validated by comparison to angle resolved photoemission spectroscopy (ARPES). We then study bilayer interfaces of the three semiconductors with each other and with Al, as well as trilayer interfaces with a varying number of ZnTe or CdSe layers inserted between InAs and Al. We find that 16 atomic layers of either material completely insulate the InAs from metal induced gap states (MIGS). However, ZnTe and CdSe differ significantly in their band alignment, such that ZnTe forms an effective barrier for electrons, whereas CdSe forms a barrier for holes. Because of Fermi level pinning in the conduction band at the interface, only electron transport is relevant for InAs-based Majorana devices. Therefore, ZnTe is the better choice. Based on the results of our simulations, we suggest conducting experiments with ZnTe barriers in the thickness range of 6-18 atomic layers.

Phase transitions, Dirac and Weyl semimetal states in Mn1−xGexBi2Te4

Using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), an experimental and theoretical study of changes in the electronic structure (dispersion dependencies) and corresponding modification of the energy band gap at the Dirac point (DP) for topological insulator (TI) have been carried out with gradual replacement of magnetic Mn atoms by non-magnetic Ge atoms when concentration of the latter was varied from 10% to 75%. It was shown that when Ge concentration increases, the bulk band gap decreases and reaches zero plateau in the concentration range of 45–60% while trivial surface states (TrSS) are present and exhibit an energy splitting of 100 and 70 meV in different types of measurements. It was also shown that TSS disappear from the measured band dispersions at a Ge concentration of about 40%. DFT calculations of band structure were carried out to identify the nature of observed band dispersion features and to analyze the possibility of magnetic Weyl semimetal state formation in this system. These calculations were performed for both antiferromagnetic (AFM) and ferromagnetic (FM) ordering types while the spin-orbit coupling (SOC) strength was varied or a strain (compression or tension) along the c-axis was applied. Calculations show that two different series of topological phase transitions (TPTs) may be implemented in this system, depending on the magnetic ordering. In the case of AFM ordering, the transition between TI and the trivial insulator phase passes through the Dirac semimetal state, whereas for FM phase such route admits three intermediate states instead of one (TI—Dirac semimetal—Weyl semimetal—Dirac semimetal—trivial insulator). Weyl points that form in the FM system along the direction annihilate when either the SOC strength decreases or a sufficient tensile strain is applied, which is accompanied by the corresponding TPTs. Model calculations of the influence of local magnetic ordering in AFM were carried out by alternating Mn layers with Ge-doped layers and showed that the magnetic Weyl semimetal state in this system is reachable at a Ge concentration of approximately 40% without application of any external magnetic fields.

Unconventional broadening of Rashba spin splitting in a Au2Sb surface alloy with periodic structural defects

Most Rashba spin splitting experimentally studied so far has ideal lattice with inversion symmetry broken, which limits the possibility to minimize the presence of spin-degenerate carriers near the Fermi level. Here, we report a novel 2D Au2Sb surface alloy decorated with periodic structural defects that exhibits modulation on the Rashba spin-orbit coupling band. Spin- and angle-resolved photoemission spectroscopy reveals a Rashba spin-split band with antiparallel spin polarization, significantly broadened compared to the Au₂Sn surface alloy. From the good agreement between the experimental results and DFT calculations, we identify that the broadening of the Rashba bands comes from variations in Sb atom corrugation induced by the periodic three-pointed star-shaped defects. These periodic defects can shift the energy position of the Rashba bands without breaking the in-plane rotational and mirror symmetries. Our findings highlight the potential to tune spin-dependent properties in 2D materials for spintronic applications.

Coexistence of Kondo effect and non trivial Berry phase in Gd doped Bi2Se3: an ARPES and magneto-transport study

Abstract The presence of magnetic impurities in Topological Insulators (TI) can disrupt their Time Reversal Symmetry (TRS) and lead to the emergence of an energy gap. This study delves into the energy band structure and the Kondo effect through the introduction of Gadolinium(Gd) magnetic perturbations (at levels of x = 0.1, 0.16) into a pure Bi2Se3 single crystal. In the case of the Bi1.9Gd0.1Se3 (5%) single crystal, the Kondo effect becomes observable at temperatures below 50K. However, the unaltered parent and Bi1.84Gd0.16Se3 (8%) exhibit typical metallic behavior. The pure sample displays the highest magnetoresistance (MR) of around 225% and demonstrates quantum oscillations driven by a nontrivial berry phase. The sample doped with 5% Gd undergoes a transition from negative magnetoresistance (NMR) to positive magnetoresistance (PMR) due to a presence of mixed magnetic state resulting from the opening of a gap at the Dirac point. This gap opening is confirmed through Angle-Resolved Photoemission Spectroscopy (ARPES) measurements. Thermoelectric properties are assessed across all prepared samples. The undoped sample displays the highest Seebeck coefficient and power factor (PF) values of -398.02 µV/K and 6.83 mW/mK2, respectively, at room temperature. These values are notably high for thermoelectric applications at room temperature.